Olefin polymers are applied to various uses as thermoplastic resin materials or modifiers of thermoplastic resins. As a polymerization catalyst used in the preparation of olefin polymers, a titanium catalyst or a metallocene catalyst is known. In the case of using the titanium catalyst, however, there is a problem that the composition of olefin types of an olefin polymer that can be prepared is restricted or a problem that the compatibility is not homogeneous because of a wide molecular weight distribution. In the case of using the metallocene catalyst, copolymerizability of α-olefins is excellent and α-olefins can be polymerized with a wide range of composition, but when high-temperature polymerization is carried out, the molecular weight is not increased or the polymerization activity is sometimes low, and this is an obstacle to low cost.
Recently, it has been disclosed by J. A. Ewen, et al. that when a catalyst comprising a transition metal catalyst having, as a ligand, isopropylidene(cyclopentadienyl)(9-fluorene) wherein cyclopentadiene and fluorene are crosslinked by isopropylidene and aluminoxane is used, polypropylene of high tacticity having a syndiotactic pentad fraction of more than 0.7 is obtained (non patent literature 1).
Moreover, it has been reported that by the use of a catalyst having a structure analogous to that of the above transition metal catalyst showing a syndiotactic polypropylene activity, a propylene/ethylene copolymer having a high molecular weight is obtained (patent literature 1). In this transition metal catalyst, however, there is yet room for improvement in polymerization performance at high temperatures and particularly in molecular weight (patent literatures 2 and 3).
By the way, a method for polymerizing an α-olefin by mixing different metallocene compounds for the purpose of allowing the resulting polymer to exhibit specific properties instead of widening a molecular weight distribution of the polymer has been also proposed. For example, a process for preparing polypropylene by using a crosslinked monocyclopentadienyl hetero atom-containing compound and an uncrosslinked biscyclopentadienyl-containing compound has been reported (patent literature 4). Moreover, there is a literature in which use of a hafnium metallocene type catalyst compound of a crosslinked bulky ligand and a zirconium metallocene type catalyst compound is discussed (patent literature 5). Furthermore, a process for preparing a propylene polymer using at least two different zirconium metallocene type catalyst compounds having a crosslinked bulky ligand has been disclosed. One of sterically firm zirconocenes used herein has, as a bulky ligand, an indenyl ligand having a substituent on a six-membered ring (patent literature 6). Still furthermore, a process for preparing polypropylene using two different zirconium metallocene type catalyst compounds having a crosslinked bulky ligand has been disclosed (patent literature 7).
The present applicant has already reported that a polypropylene polymer having a relatively high melting point and a high molecular weight can be prepared by the use of a specific transition metal catalyst (patent literature 8).